I'm dubious about the information DH supplied about Mulciber in school. That he used "Dark Magic" on Mary McDonald.
McDonald was a Gryffindor; they think the worst of Slytherins almost naturally. What if the most Mulciber had actually done was Hex her? In a Gryffindor's eyes, a Hex in the hand of a Slytherin might as well be "Dark Magic."
If it was just a Hex, that would explain why Snape brought up Potter and his friends to Lily. Maybe it wasn't because he didn't like Potter, but was because Potter and Mulciber were very similar.
Although to be fair, technical "Dark Magic" does get thrown around at that school on occasion. I think it should depend on the severity of the Curse-and-or-Hex itself, more than on the specific type of magic used, how bent out of shape one gets about it.
I think the boundaries between types of magic are, in most cases, pretty fluid. I mean, I'm obviously not arguing that the Killing Curse or Sectumsempra or the other Unforgivables are anything but Dark Magic -- but where does one draw the line between a hex and a jinx, for example? Something that may technically be Dark Magic could be used for nothing more than a prank, while something that's called nothing more than a Jinx could cause a lot of damage in the wrong (right) hands.
Then again...I'm a Slytherin.
Frankly, I'm dubious about all information supplied by the books; one has to remember that they are all experienced, almost every single chapter, through Harry's eyes, and take that biased perspective into account when filtering said information.
I mean, let's face it: do we really think Draco Malfoy is not so shallow and arrogant, that he would take an ugly girl to the Yule Ball? No. Pansy obviously is at least cute; she just never gets described that way because it's Harry looking at her. I'm sure if one of Pansy's friends were the main POV she wouldn't be called "pug-faced," she'd be called "button-nosed." For example. Likewise, I'm sure if Ron were the POV, Hermione's descriptions would appear more flattering, at least as the books go on (less "shrill voice" and "bushy hair" I'd think). It's all via Harry, right down to the most basic facts which we encounter. Look at how Sirius's descriptions evolve as Harry's opinion of him changes (although I'm sure his appearance actually does improve between PoA and OotP, obviously!)
If I recall correctly, JKR even lampshades this in Stone during the sorting, when Harry looks over at the Slytherin table and wonders if the kids there look unpleasant, or if he's just seeing that because of what he's heard about that house's reputation. So...yeah. Filtered.